Sheathed electric heating elements



Jan. 7, 1958 R. L. FEARN SHEATHED ELECTRIC HEATING ELEMENTS Filed Aug. 16, 1952 United States Patent SHEATHED ELECTRIC HEATING ELEMENTS RichardLee Fearn, Homer-City, Pa., assignor to Syntron Company, Homer City, Pa., a corporation of Deltaware Applicati'on'August16, 1952, Serial No. 304,785

This invention relates generally to electrical heating units and more particularly to the process of making electrical hegting, units, wherein the heating conductor is electrically insulated and thermally conductive within a tubular sheath.

The electrical heating unit comprising. this invention ispreferably made by the use of a ribbon formed from a sinuous heating conductor which may be constructed of wirethat is flat or round in cross-section,-.but is bent into'a sinuous ribbon so as to provide a more etlicient distribution ofheat throughout the unit. Other types ofribbon conductors may be employed. The ribbonis assembled to electric terminals which may be silver soldered or other-wise fastened to the ends of the ribbon heating conductor; This assembly is then enclosed within a suitable fabric or cloth such as asbestos cloth, or a silicon dioxide fabric made of leached glass-like filaments placed on both sides of theribbon andterminals and is then inserted between two pieces of flexible metal tape. The silicon dioxide cloth is made into an improved thermal conducting and electrical insulator by pressure. When stacked in this manner, one is enabled to feed or draw the two strips of flexible tape through a flattened the assembly. The flexible tapes are then withdrawn one at a time.

In order to insure that theassembly will not be disturbed when each flexible tape is withdrawn, it is desirable to have the tubular sheath flattened sufliciently to offer some resistance to the initial drawing of the assembly into the tubular sheath. And, at the same time, it is desirable toprovide' guide fingers to engage'the edges ofthe ribbon that is covered by the silicon dioxide cloth as to properly centerthe ribbon with respect tofthe tapes as the assembly is' drawn into the tubular sheath. In this manner, one can be assured that the heating" ele ment will'be" roperly'lbcated' within the tubular sheath.

Upon withdrawing the flexible tape, it is necessaryto Withdraw one tape at-a time, so asnot to disturb the assembly. It'makes' little difference whether'the' upper or lower flexible tape is withdrawn, so long as one is M removed at a time; The tapes'aid -in'retaining the proper location of the ribbon within'the' tubular sheath. However, it is preferable to'crease'the center of the sheath in order to insure that the tubularsheath grips the cloth covered ribbonand'holds the same during. a subsequent flattening operation.

The cloth employedas an insulating material is preferably a glass cloth Known as" silicon dioxide cloth made of a boron glass. This cloth has the property of b eing made up inthreadsthat are woven-into an-ordinarys cloth that' looks like -a piece of goods for usein-curtains, suits, or other similar: garments. This cloth is then leached to remove a majority of the elemen tsmaking:up the cloth other than silicon dioxide; After'tlieassembly isthre'aded into the tubular sheath,the"tubular sheath iswsubjectedto Patented Jan. 7, 1953 2 pressure to pulverize the silicon dioxide cloth and compact it into a dense mass. This may be performed by pressing the whole length of the tubular sheath simultaneously, or by passing the same through rollers which exert sufllcient pressure to pulverize the cloth and convert the silicon dioxide into a powdered and dense mass.

This setup converts the glass clotn from a spacing agent to an electrical insulating agent that is thermally conductive. The glass cloth has thermal insulating properties which are converted by this pressure step into thermal conducting properties.

The pressure employed to compress the tubular sheath on the assembly of the ribbon between the silicon dioxide cloth layer should be sufiicient to transform the silicon dioxide cloth into powder and substantially fill all the voids within the tubular sheath. Obviously the sheath must not be very much wider than the ribbon. However, it should be sufiicient to permit the insertion of the cloth beyond the edges of the ribbon. It has been found that a very good electrical insulating condition will be performed, and the heating unit may be bent along its greater axis without endangering an electrical connection between the ribbon and the tubular sheath.

When mica plate is employed to insulate the ribbon, it is necessary to interlock the ribbon between the layers of mica. It is preferable to employ a sinuous ribbon that has been constructed so that a portion intermediate of each bend is slightly flattened'or indented so as to permit interengagement' between the mica and the ribbon for holding the same in assembled relation when positioned in stacked assembly between the layers of mica.

In order to feed the assembly, through the tubular sheath, it is advisable to make the strips of flexible pull tapes longer than the assembly, so that they extend through the sheathin front of the assembly, including the terminals and the ribbon. These pull strips must thus be more than twice the length of the sheath. Upon threading the a'ssembly, into the tubular sheath, it is preferable'to have some sort of a guide for holding the whole assembly in alignment as it is being drawn into the tubular sheath.

However, the heating element may have both terminals at one end, the ribbon being doubled back on itself and provided with another layer of insulating cloth therebe tween; and they both are then drawn into the sheath at the same time.

A heating unit of this character provides a very small amount of insulating material between the ribbon formed of the sinuous heating conductor and the tubular sheath. The spacing between the'ribbon and the sheath can be maintained uniform, and at a minimum, due to the uniformity of distribution of the electrical insulating material. The unit can be bent into different shapes or in circular or spiral shapes to form different types of heaters Without causing an electrical connection between the heating element and the tubular sheath because of this uniform distribution of the electrical insulating material. This advantage, together with the use of a flat ribbon, provides'a heating unit thatwill heat up materially faster than anything known in-the prior art, and provides the unit that will cool faster and transfer a greater amount of the heat at a materially faster rate, and thus provide more eflicient heating than any heating element of this type.

Other objects and advantages appear hereinafter in the following description and claims.

The accompanying drawing shows for the purpose of exemplifi'cation, without limiting the invention and claims thereto, certain practical embodiments of the invention wherein:

Fig. l is a schematic perspective view illustrating the J heating unit assembly placed in position for drawing into the tubular sheath.

Fig. 2 is an exploded sectional view of the assembly such as shown in Fig. 1.

Fig. 3 is a cross-sectional view showing the center crease in the heating unit.

Fig. 4 is a cross-sectional view of the heating unit when completely compressed, and having a terminal attached thereto.

Fig. 5 is an enlarged view of the sinuated ribbon illustrating the indented portions adjacent the bend of the sinuated conductor to be employed with a mica insulation.

Fig. 6 is a view in side elevation of the structure shown in Pig. 5.

Referring to the drawings, and particularly to Figs. 1 and 3, the heating element is made up of the sheath member such as indicated at 1 which is partially flattened as shown, and which is held in clamped relation by the hold down members 2 supported on the base 3. The edges of the sheath abut the hold down plates 2, and the sheath ends engage the abutments 4 and 5.

The base 3 is preferably materially longer than the assembly, and is provided with a series of guides such as illustrated at 6 which are removably placed in position to loosely hold the assembly and permit the same to be made up before it is drawn into the tubular sheath 1.

The assembly comprises the bottom tape 7 resting on base 3. A bottom layer of cloth 8 rests on the strip 7 and the sinuous ribbon 9 made of electrical resistant maetrial and is covered by the top layer of cloth as indicated at 10 and the top flexible tape as indicated at 11. The flexible tapes 7 and 11 extend through the sheath and are materially longer than the assembly which starts as shown at 12, in Fig. 1, wherein the conductor, as indicated at 13, is laid between the upper and lower strips 11 and 7, and has been attached to the end of the ribbon, as indicated at 14. These draw tapes are preferably in the order of three thousandths of an inch thick.

The upper and lower tapes 11 and 7 are gripped at their free ends and provided with a pulling shackle 15 which may be grasped by hand to pull the assembly into the sheath, or attached to a machine to provide a uniform drawing force that moves the assembly into the tubular sheath with uniform force and speed.

The opposite end of the ribbon is likewise attached to a suitable terminal in the same manner as that previously described when the unit is supposed to have a terminal at both ends thereof.

A pair of guiding fingers, such as indicated at 16 are employed to extend between the tapes, and insure that the edges of. the ribbon are uniformly faced between the tape. These guide fingers bear against preferably two thicknesses of the cloth. When the complete assembly has been drawn within the tubular sheath, and the very ends of the terminals extend therefrom, wedges of mica may be inserted to act as plugs for the powder and properly space the terminals from the sheath, and thereby insulate the same as shown in Fig. 5. However, these mica plugs 17 may not provide a complete gas seal, as it is desirable that the tube be permitted to breathe.

As shown in Fig. 4, the metal sheath 1 has been sub-- jected to pressure to convert the cloth, such as indicated at 8 and 10, into powder as indicated at 18. This powder provides a minimum thickness of electrical insulation and thermal conduction between the sheath and the sinuous ribbon 9, yet it is ample for this purpose and reduces the thermal efficiency for transferring the heat between the ribbon and the sheath.

The sinuous wire ribbon 9, as shown in Figs. 5 and 6, is indented as indicated at 20, by the grip when the bend is made therein to form the ribbon. This indented 0r flattened section 20 produces the shoulder portions 21 which aid in preventing the ribbon from slipping laterally after it has been inserted between mica plates in the sheath.

As shown in Fig. 4, before the whole of the sheath is flattened to produce the structure as shown at Fig. 4, the cloth members 8 and 10 are still intact when the center of the sheath is depressed, as indicated at 22, for the purpose of increasing the pressure on the assembly before the whole of the sheath is subjected to pressure as by rolling, otherwise, shifting of the assembly might occur.

The whole of the assembling process comprises the steps of first attaching the electric terminals to the heating conductor ribbon, and then forming the stacking by placing the flexible strip 7 on the base 3 between the guide 6 and then placing the strip of cloth 8 made of silicon dioxide on the strip, then setting the conductor ribbon 9 on the cloth 8 in the proper position relative to the cloth. The second strip of cloth 10 is then placed over the ribbon element 9 and the flexible tape 11 is placed on top of the cloth 10 to complete the assembly. The cloth may be made by folding over one edge, however, it is preferable to use two strips of cloth and let the ends of the cloth lie over the ends of the ribbon 9 so as to provide two thicknesses between the ribbon and the wall.

The strips 7 and 11 are permitted to extend materially beyond the assembly, so that it can be threaded through the tube 1 which is clamped in position, and provided with an attachment for puller mechanism such as indicated at 15. The assembly is then drawn past the guiding fingers 16 which insure the centering of the assembly before it enters the tube 1, and as it is being drawn into the flattened tubular sheath 1, the latter has been partially, sutficiently flattened to create a pressure on the assembly so that it will not move or otherwise deviate from the position placed on it by the guiding fingers. This resistance by the tubular sheath insures that the same to be properly assembled before the center is creased as indicated at 22. The tubular sheath 1, as show-n in Fig. 4, may be creased on one or both sides as may be desired, however, this increased pressure prevents any movement of the assembly laterally when it is subjected to suflicient pressure to powder or convert the silicon dioxide cloth to powder, as previously stated, which may be accomplished by placing the whole of the length of the strip under a press or by rolling the same between pressure rollers which progressively flatten the tube and crushes the cloth to convert it into powder and thence into a solid mass, yet does not cause the same to become deformed by reason of the pressure rollers. Before the pressure is applied to insert the crease 22, it is preferable to remove one tape at a time from the assembly.

For clarity of explanation, certain embodiments of this invention have been shown and described. It is to be understood that this invention is capable of many modifications and changes and the construction and arrangement of parts may be made therein, and certain parts may be employed without the conjoint use of other parts and without departing from the scope and spirit of this invention. 1

I claim:

1. The process of making an electrical heating conductor comprising the steps of attaching electric terminals to the heating conductor ribbon, stacking the conductor between layers of a leached glass fabric backed with flexible metal tape to form an assembly, threading the stacked assembly through a partially flattened tubular sheath which offers some resistance, withdrawing the flexible metal tape leaving the heating conductor sandwiched directly between layers of the leached glass fabric within the tubular sheath, and applying pressure on the tubular sheath to convert the leached glass fabric into a powdered mass.

2. The method of claim 1 characterized in that the tubular sheath is flattened sufliciently to otter resistance to the threading of the assembly therethrough to keep the ribbon and fabric properly positioned.

3. The process of claim 1 which also includes the step of guiding the stacked assembly as it enters the tubular sheath by engaging the sides thereof to maintain it centered.

4. The process of claim 1 which also includes the step of longitudinally creasing the center of the partially fiattened tubular sheath before applying pressure thereto to hold the heating conductor and the covering layers of cloth centered in the tubular sheath.

5. The process of claim 1 characterized in that the metal tapes of the assembly extend beyond the ribbon for threading through the tubular sheath before the complete assembly enters the tubular sheath.

6. The process of claim 1 characterized in that one tape is removed at a time when the assembly is properly disposed in the tubular sheath.

7. The process of claim 2 characterized in that the ribbon conductor is slightly flattened inwardly of each bend which holds and locks the ribbon in place between the layers of the electrical insulating material when subjected to the pressure exerted by the tubular sheath.

8. The process of converting a porous leached glass fabric spacing material but not electrically insulating material of high thermal insulating ability into a high electrical insulator with excellent dielectric qualities and excellent thermal conductivity while the material is retained within a containing sheath which comprises applying pressure to the sheath of sufficient magnitude to crush the material.

9. The process of making an electrical heating conductor comprising the steps of attaching electric terminals to the heating conductor ribbon, stacking said conductor between wider layers of asbestos fabric backed with flexible metal tape to form an assembly, threading the stacked assembly through a partially flattened tubular sheath which otters some resistance to movement of the assembly therethrough, withdrawing the flexible metal tape leaving said heating conductor sandwiched directly between layers of the asbestos fabric with the tubular sheath, and applying pressure on the tubular sheath to compress the ashestos fabric into a dense and hard mass contacting the walls of the tubular sheath with the conductor i-mbedded in the center thereof.

10. The process of converting a porous asbestos fabric spacing material but not an electrical insulation into a high electrical insulation with improved dielectric qualities and excellent thermal conductivity while the material is retained within a metal sheath which comprises stacking a conductor between wider layers of asbestos fabric, threading the stacked assembly through a tubular sheath, and applying pressure to the sheath of suflicient magnitude to convert the asbestos fabric into a dense and hard mass confined within the sheath with the conductor imbedded in the center thereof.

References Cited in the file of this patent UNITED STATES PATENTS 1,093,792 Madsen Apr. 21, 1914 1,323,220 Heiser Nov. 25, 1919 1,359,400 Lightfoot Nov. 16, 1920 1,677,240 Kochendorfer et al July 17, 1928 1,913,355 "Wiegand June 9, 1933 2,012,977 Trent et a1. Sept. 3, 1935 

